1. Field of the Invention
The present invention relates to an OCB (Optically Compensated Bend) mode liquid crystal display and a driving method thereof, and more particularly to an OCB mode liquid crystal display which can rapidly shift the orientation state of liquid crystal molecules from an initial orientation state into an orientation state for screen display while consuming low power, and a method for driving the same.
2. Description of the Prior Art
As generally known in the art, liquid crystal displays are light, thin, short and small, are driven by a low voltage, and consumes a small power, so that the liquid crystal displays are now replacing cathode ray tubes (CRTs). Particularly, a thin-film transistor liquid crystal display has been a high-quality, a large size and colorization equal to those of the CRT, so that the thin-film transistor liquid crystal display is being variously utilized in many fields. Such a liquid crystal display includes an array substrate on which thin-film transistors and pixel electrodes are mounted, and a color-filter substrate on which a color filter and corresponding electrodes are mounted. The array substrate and the color-filter substrate are attached to each other with a liquid crystal layer interposed between the substrates. Generally, the liquid crystal display utilizes twisted-nematic (TN) mode liquid crystal.
A TN mode liquid crystal display has a high contrast in image display but has strong viewing-angle dependence. In order to solve such viewing-angle characteristic of the TN mode liquid crystal display, various methods including a pixel area division technique have been proposed. Also, to this end, an in-plane switching (IPS) mode liquid crystal display and a technique of forming a double domain in a liquid crystal layer have been proposed. Herein, the technique of forming a double domain in a liquid crystal layer includes a multiple rubbing method, a multiple orientation method, an edge fringe field method and a parallel fringe field method.
However, the conventional liquid crystal displays manufactured by the above-mentioned methods still have a problem in that they have a low response speed. That is, since the liquid crystal display utilizing the TN mode liquid crystal has a low response speed (the response speed between gradation displays is 100 ms in maximum), it is impossible to achieve 16.7 ms required to display a high-speed moving picture. Therefore, it has been required to develop a liquid crystal display having a wide viewing angle and a high response speed enough to display a high-speed moving picture. As a result, an OCB (Optically Compensated Bend) mode liquid crystal display has been proposed in order to improve a response speed and to obtain a uniform viewing-angle characteristic in all directions.
Hereinafter, the construction and the operation of a conventional OCB mode liquid crystal display will be schematically described with reference to FIGS. 1A to 1C.
The conventional OCB mode liquid crystal display contains: an upper substrate 110 including a color filter, an upper driving electrode 140 and an orientation layer; a lower substrate 120 including a TFT (Thin Film Transistor), an lower driving electrode 150 and an orientation layer; and liquid crystal molecules 130 interposed between the upper and lower substrates 110 and 120. In this case, all the orientation layers included in the upper and lower substrates 110 and 120 are aligned in the same direction to each other. In the OCB mode liquid crystal display having such a construction, when a voltage is not applied between the upper and lower electrodes 140 and 150, the liquid crystal molecules 130 are, as shown in FIG. 1A, maintained in a splay orientation state (which is an initial orientation state) according to the orientation processing direction of the orientation layers.
Meanwhile, when a pre-tilt voltage is applied between the upper and lower electrodes 140 and 150, the liquid crystal molecules 130 are shifted from the splay orientation state into a bend orientation state as shown in FIG. 1B. In this case, the pre-tilt voltage must be higher than a transition voltage which is required for the liquid crystal molecules 130 to start a transition from the splay orientation state to the bend orientation state. A time period required for the liquid crystal molecules 130 to start a transition from the splay orientation state to the bend orientation state is called “transition time”. When a driving voltage is applied between the upper and lower electrodes 140 and 150, the liquid crystal molecules 130 are shifted from the bend orientation state into a vertical orientation state by the driving voltage as shown in FIG. 1C, thereby linearly transmitting light. Thereafter, when there is no applied voltage, the liquid crystal molecules 130 are again shifted into the splay orientation state.
As described above, according to the conventional OCB mode liquid crystal display, the sate of the liquid crystal molecules 130 is repeatedly shifted in the sequence of the splay orientation state, the bend orientation state and the vertical orientation state by the voltage applied between the upper and lower electrodes 140 and 150, thereby displaying an image. In this case, a pre-tilt voltage is first applied and then the driving voltage is applied so as to display an image.
However, such a conventional OCB mode liquid crystal display requires lots of power for the pre-tilt voltage to shift the liquid crystal molecules from the splay orientation state into the band orientation state, thereby increasing power consumption. Also, the conventional OCB mode liquid crystal display has problems in that the transition speed from the splay orientation state to the bend orientation state is slow (in fact, the transition between the two states is an unnecessary step in the process of displaying an image), its driving voltage is high, and it is difficult to achieve the bend orientation.
In order to solve such problems, there has been proposed a method for holding the liquid crystal molecules in a high pre-tilt state by mixing a monomer or a UV curing agent with liquid crystal molecules so as to easily achieve a transition to the bend orientation. As another method to solve the problems, Korean Patent Laid-Open Publication No. 2002-0097025 discloses a method for forming slant of a continuous saw-tooth shape on a cell surface so as to use the slant surface as a nucleus to shift liquid crystal molecules into the bend orientation state.
However, the former method has a problem in uniformity of display and the latter method has a difficulty in forming a slant surface on an ITO layer. Therefore, it is difficult to actually use either of the methods.